Variable attenuators



Dec. 25, 1962 A. M. RESLOCK 3,07

VARIABLE ATTENUATORS Filed Oct. 10, 1960 3 Sheets-Sheet 2 xwmxxxxw 9 aw gamma I INVENTOR. AZ FREfl/W. R 551 06K Dec. 25, 1962 Fiid oct'. 10, 1960 .HG/VAL GENERATOR A, M. RESLOCK VARIABLE ATTENUATORS 3 Sheets-Sheet 3 SIGNAL AUENUA TOR 0U TPUT' OUTPUT FIG. 6

SIG/VAL GENE/M 70R nrrs/vzm ran 5'0 .72 purPuT R son INPUT INVENTOR. ALFRED/14. A SLCK ATTORNEY This invention relates to variable attenuators, especially for the VHF range.

One of the objects of the invention is to provide a variable attenuator for a frequency range between a few megacycles up to about 500 megacycles of compact construction, high efficiency and easy adjustability and accuracy.

A more specific object of the invention is to provide a number of substantially equal impedances in a substantially cylindrical cavity forming a circular array along the wall of said cavity, and to divide the cavity by means of separating walls into a number of substantially electrically separated, equal sectorial compartments with the impedances arranged straddling adjacent compartments and traversing the corresponding walls in a direction substantially perpendicular to these walls.

Another object of the invention is to arrange the impedances in a circular array within a substantially cylindrical space along the wall of the space straddling radial walls of a number of electrically separated compartmental sectors and to provide input and output connectors extending respectively, in a direction of the axis of the cavity and in a direction substantially perpendicular to that axis.

Still another object of the invention is to divide a substantially cylindrical cavity into a number of substantially equal, electrically separated sectorial compartments, in which the walls of these compartments are straddled or traversed by impedances forming the adjustment steps of the attenuator and to vary adjustment by rotating a member coaxially with the axis of the cavity from one of these impedances to another.

Still another object of the invention is to provide in a cylindrical cavity a number of substantially equal, electrically separated compartments having radial planar walls which are formed of two adjacent portions supplementing each other, one being fixedly attached to the cavity and the other being moveable with and attached to a cover plate closing the cavity.

These and other objects of the invention will be more fully apparent from the drawings annexed herewith in which FIGS. 1 and 2, in top and side views, respectively,

both in cross-section, show a cavity attenuator embodying certain principles of the invention.

FIGS. 3 and 4, respectively, show the parts of the cavity attenuator in a disassembled position; FIG. 3 represents the bottom cavity of this structure; and FIG. 4 the top cavity or cover plate.

FIG. 5 represents a circuit diagram for a cavity attenuator according to FIGS. 1 and 2; and FIGS. 6 through 8 illustrate different application circuits for such a cavity attenuator.

FIGS. 9 and represent circuit diagrams applying the invention to higher input and output impedances.

As apparent from FIGS. 1 and 2, a cavity attenuator is shown to consist of a cylindrical metallic wall having a bottom plate 2, attached thereto and a preferably removable top or cover plate 3. Attached to bottom plate 2 and also, if necessary, to wall 1 are a number of radial metal or shielding plates 5, covering approximately half of the radial cross-sectional space of metal wall 1, and dividing the cylindrical cavity into a number of compartments or sectors corresponding to the number of attenuaatfi a Q@ 3,070,763

Patented Dec. 25, 1962 tion steps desired, plus at least two additional walls 6 representing one empty compartment and providing a separation or insulation between maximum and minimum attenuation. Supported on and straddling these separation walls 5, there is arranged a circular array of series connected resistances 7 of a number, in the present case ten, corresponding to the desired adjustment steps of the attenuator.

In addition, there are provided a number of shunt resistors 8 connecting, in an inclined position with respect to the cavity axis or bottom plate 2, the ends of resistors 8 to bottom plate 2 or ground.

The other halves of shielding walls 5 and 6 designated by 5 and 6' (not shown), respectively, are attached to the top plate 3 and movable therewith, if necessary, for assembly and disassembly and, if necessary, adjustment of the cavity structure.

Input and output terminals are provided, in accord ance with the invention and as apparent from circuit diagram of FIG. 5, at, the center and at the periphery respectively, of structure 1, 2., 3.

More specifically, in a preferred embodiment of the invention input is applied at the center by way of a coaxial input connector 9 attached to the top plate 3 and movable, if necessary, together with top plate 3 and walls 5, 6' attached thereto. A wiper 10 is pressed by center spring '11 against the inner conductor 9 of the axially arranged input connector 9. Wiper 10 extends radially through a slot 12 of rotor 13 and is rotatable therewith. Rotor 16 may be controlled in its angular position through opening M in any desired manner without affecting the scope of this invention.

During rotation of rotor 13 wiper 1t) glides in conductive contact with, and between, a plurality of pairs of contact springs 14, 15 which are attached in a circular array to insulating disc or deck 16 supported in cut-outs 17 of shielding walls 5, 6. Contact springs 14, 15 are electrically connected to junction points 18 of series rcsistances 7 with shunt resistors 8 and, at the same time, over line 19 to inner conductor 21) of the radially arranged output connector schematically indicated in FIG. 1 at 20.

Thus rotation of output wiper 1% under control of rotor 13, as schematically indicated in the circuit diagram of FIG. 5, from one junction point 18 to an adjacent junction point, will vary the available output attenuation by 10 db or altogether between 0 db and db.

At the same time, outer conductor 26 of the radial output connector 20, 20' and outer conductor 9 of the axial input connector 9, 9' are interconnected and both are connected to housing 1, 2, 3, or ground.

Similarly, rotor 13 may be connected to ground through outer conductor 9 and a circular spring, schematically indicated in FIG. 1 at 21.

The attenuator according to the invention may be used in signal generators, where the RF voltage is developed at a comparatively high impedance and stepped down in order to obtain isolation of the output from the oscillator. The input and output impedances of the attenuator have been found to be perfectly constant, regardless of attenuation, when the attenuator is driven from high impedance. When driven from a lower impedance source, its output impedance is modified by the product-over-sum of the driving impedance and the characteristic impedance of the attenuator (in low attenuation positions).

In the case where a 50 ohm attenuator is connected to a 50 ohm output of a signal generator, the attenuator appears as a 50 ohm resistor shunting the output. The equivalent circuit appears as in FIGURE 6.

In the case of the conventional toggle switch type attenuator which adds or removes separate 11' type attenuators, the circuit would appear as in FIGURE 7. However, unless this type is terminated, the impedance looking back from the output will vary as the attenuator is varied. Therefore, it must be terminated. When this is done, FIGURES 6 and 8 looking back into the attenuator (and generator) are identical. In this case, the usual 6 db loss is experienced when the attenuators are connected to the generator-assuming the generator is a constant current source. The rotary attenuator is interchangeable with a conventional attenuator (when the conventional attenuator is terminated).

It might also be noted that in FIGURES 6 and 8, the resistance (considered as a DC. circuit) is assumed to be 25 ohms. The resistance would approach 50 ohms as attenuation is increased.

Generally the design procedure for db step attenuator (input and output impedance=50 ohms would be as follows:

(1) Design a basic 1r attenuator for twice the characteristic impedance (100 ohms) required and for the attenuation required per step (10 db).

(2) Each shunt resistor of the attenuator is composed of 2R in parallel, hence the actual resistor is R/2.

(3) The end resistors are composed of R in parallel with twice the characteristic impedance.

In an example where Z=input and output impedance=lOO ohms (2X50 ohms) and N=power ratio: (10 rib/setup) the circuit shown in FIGS. 9 and 10 applying with the following calculations:

substantially cylindrical cavity, radial walls for dividing said space into a number of substantially equal sectorial compartments electrically separated from each other, each of said radial Walls consisting of at least two adjacent portions supplementing each other along a substantial linear separation line, substantially equal resistors serially connected and supported on each of said wall portions along said separation lines in a position traversing said separation lines; said resistors forming a substantially circumferential array substantially coaxial with the axis of said cavity and along the wall of said cavity; and further comprising a cover plate at one end of said cavity and input and output coaxial connectors arranged respectively at right angles to each other and attached to said cavity wall and said cover plate respectively.

2. Attenuator according to claim 1 comprising at least two empty compartments provided between the first and the last one of said sectorial compartments containing said resistors.

3. Attenuator according to claim 1 wherein said two coaxial connectors are substantially equal, at least one of said connectors extending radially and perpendicularly to the side wall of said cavity and one other of said connectors extending radially and perpendicularly to the top of said cavity.

4. Attenuator according to claim 3 comprising a rotary member arranged coaxially in the center of said cavity coaxial with the input connector, said input connector having an inner conductor and said rotary member having a wiper radially extending thercthrough; and a center spring supported on said rotary member and pressing against the inner end of said wiper while said wiper is pressed by said spring against said inner conductor so as to retain contact when rotor and wiper are rotated about the axis of said cavity, and a circular array of stationary contacts supported on said separating walls and arranged in the path of said Wiper at the other end thereof and connected to the junction points of adjacent series resistors.

5. Attenuator according to claim 4 comprising a number of shunt resistors arranged in inclined positions with respect to the axis of the attenuator and connecting the junction points of said series resistors to one of said end plates.

6. Attenuator according to claim 1 wherein said input and output connectors have outer conductors connected to wall portions of said cavity and inner conductors interconnected by a central spring arranged in the center of said cavity, comprising a wiper rotatable about the cavity axis and contacts arranged on a stationary deck coaxial with said cavity and connected to junction points of said series resistors.

7. Attenuator according to claim 1 comprising the inner conductor of a coaxial connection and circular arrays of stationary contacts insulatingly supported on said separating walls and of series connected resistors having junction points connected to said contacts and means including a wiper and connector for connecting said stationary contacts selectively to said inner conductor.

8. Attenuator according to claim 1 comprising a circular array of series resistors arranged in a plane perpendicular to the cavity axis, and an array of shunt resistors arranged along a conical surface formed about said axis and connected between junction points of said series resistors and portions of the cavity wall.

References Cited in the tile of this patent FOREIGN PATENTS 463,443 Canada Feb. 28, 1950 

